6276: Extract call_info and completion into separate crates r=matklad a=popzxc
As it was discussed in [zulip](https://rust-lang.zulipchat.com/#narrow/stream/185405-t-compiler.2Fwg-rls-2.2E0/topic/Completion.20refactoring), we need to move `completions` into a separate crate.
Unfortunately, the dependency on `call_info::ActiveParameter` doesn't look easy to get rid of, and it seems to be a topic for a separate PR, thus I also extracted `call_info` into a separate crate (on which both `ide` and `completion` crates depend).
Additionally, a few `FIXME`s in doc-comments were resolved in order to make `tidy` happy.
Co-authored-by: Igor Aleksanov <popzxc@yandex.ru>